1. Field of the Invention
This invention relates to a picture drawing apparatus and a picture drawing method used in a graphics computer, a special effect device or a video game machine which are a video equipment employing computers.
2. Related Art
In a picture generating device in a domestic TV game machine, personal computers or graphics computers, used for generating data of a picture outputted for display to a television receiver, a monitor receiver or a cathode ray tube (CRT) display device, a dedicated drawing device is provided between central processing unit (CPU) and a frame buffer for enabling high-speed processing.
That is, in the above-mentioned picture generating device, the CPU when generating a picture does not directly access the frame buffer, but performs geometry processing, such as coordinate transformation, clipping or light source calculations, defines a three-dimensional model as combination of basic unit figures, such as triangles or quadrangles, in order to formulate a drawing command for drawing a three-dimensional picture, and sends the drawing command thus generated to the drawing device. For generating, for example, a three-dimensional object, the object is resolved into plural polygons and a drawing command for each polygon is sent from the CPU to the drawing device. The drawing device interprets the drawing command sent from the CPU to execute rendering processing of writing pixel data in the frame buffer, taking into account the Z-values and colors of all pixels making up the polygon, from the Z-values specifying color data and depth of the apex points, for drawing a figure on the frame buffer. The Z-values represent the information specifying the distance from the viewing point along the depth direction.
If, for example, a three-dimensional object is displayed in the above-described picture generating device, the object is resolved into plural polygons, and a drawing command associated with each polygon is transmitted from the CPU to the drawing device. For representing the object more realistically, there are used techniques termed texture mapping or MIP mapping. There are also widely known techniques of converting picture color data via a color lookup table (CLUT) having stored therein color conversion data for changing the display colors.
The texture mapping is a technique of affixing a two-dimensional picture pattern provided separately as a texture source picture to the surface of a polygon constituting the object. The MIP mapping, on the other hand, is among the techniques of texture mapping of interpolating pixel data so that the affixing pattern on the polygon will not be non-spontaneous in case the three-dimensional model is moved towards or away from the viewing point.
Meanwhile, the picture drawing speed depends on the processing speed of texture mapping or MIP mapping for each polygon in a drawing engine. Moreover, the picture drawing speed is influenced by the writing speed from the drawing engine to the frame buffer, such that, if the frame buffer accessing speed is low, the drawing speed is lowered. Therefore, if an expensive high-speed memory is used as a large-capacity frame buffer, for increasing the drawing speed, the system cost is prohibitively increased. However, if an inexpensive dynamic random-access memory (DRAM) is used, the drawing speed of the system is lowered.
In view of the above depicted status of the art, the present invention has for its objects to provide the following.
That is, it is an object of the present invention to provide a picture drawing device and a picture drawing method whereby a high drawing speed may be maintained even with the use of an inexpensive memory, such as DRAM, as a frame buffer.
It is another object of the present invention to provide a picture drawing device and a picture drawing method whereby, in the picture drawing device for executing texture mapping by drawing means, drawing processing can be executed without stopping the drawing means.
It is yet another object of the present invention to provide a picture drawing device and a picture drawing method whereby the number of times of accessing and the accessing time of the picture memory can be decreased to raise the overall picture drawing speed.